Mixed reality is a technology that allows virtual imagery to be mixed with a real-world physical environment. A see-through, near eye display device may be worn on a user's head to view the mixed imagery of real objects and virtual objects displayed in the user's field of view. In order to facilitate the illusion of three-dimensional depth, images of virtual objects are displayed independently to the left and right eyes by the head mounted display device. Images on the left and right displays can be positioned such that the angles of virtual objects are the same as the binocular disparity created at the eyes by an object in the real world. This intentional horizontal binocular disparity (convergence or stereopsis) is closely matched to the horizontal parallax created between the horizontally displaced eyes and a virtual object at a defined distance. This binocular disparity is interpreted by the brain as indicative of a depth of the virtual object in the mixed reality environment. It is desirable to precisely control the binocular disparity of displayed images, as departures from the correct angles can cause a conflict with other visual cues such as motion parallax. These conflicts can diminish the users experience and worst case, the immersive experience can be lost. Furthermore, departures from alignment between the images in the vertical direction (vertical disparity or dipvergence) or divergent binocular disparity is unnatural and cannot be accommodated by the eye brain system. Even small deviations (for example 1-2 mRads) can create, discomfort and larger errors cannot be fused by the eye brain system at all, resulting in the virtual image to appear as a double image.
Optical display systems can be initially calibrated for accurate binocular disparity. However, typically head mounted display devices are light-weight, and can deform either under shock or when worn on a user's head. It is therefore required to detect displacement of the left eye piece relative to the right eye piece of the head mounted display with the aim of electronically or mechanically correct the displacement. In an example, a laser light source on one eye piece transmits across the nose bridge to a photo-detector array on the opposed eye piece. Relative movement of left and right eye pieces may result in a detectable change in where the laser strikes the photo-detector array to thereby indicate the relative displacement.
A shortcoming of such a detector system is that linear displacement of one eye piece relative to the other may not be discernible from angular displacement of one eye piece relative to the other. Both may result in the same measured displacement of the laser beam on the photo-detector array. While adding additional laser/photo-detector array pairs may provide additional information, there may still be some combinations of translation and/or rotation which are not discernible with such a detector system. Moreover, such a detector system only provides information regarding displacement of one eye piece relative to the other eye piece. The described conventional system is not able to provide information of absolute movement of one of the left and right eye pieces relative to a frame of the head mounted display.